This invention relates to a pollucite-based, highly refractory ceramic having high resistance to thermal shock and being suitable for high temperature applications such as filters for filtering particulates from diesel engines exhaust streams.
Pollucite which has the composition Cs2O.Al2O3.4SiO2 or CAS4 is the most refractory silicate known exhibiting a melting point above 1900xc2x0 C. However, a drawback associated with this material is its high coefficient of thermal expansion at temperatures up to 400xc2x0 C., which is usually about 120xc3x9710xe2x88x927/xc2x0 C., rendering the material a low resistance to thermal shock. Therefore, despite its high refractoriness, pollucite would not be suitable for filtering applications of gas engines exhausts such as diesel particulate filters.
It would be considered an advancement in the art to obtain a material which has the high refractoriness of pollucite and also high resistance to thermal shock.
The present invention provides such a pollucite-based material and a method of fabricating the same.
In accordance with one aspect of the invention, there is provided pollucite-based ceramic structures having high refractoriness and a high resistance to thermal shock, properties which make the inventive structure extremely desirable in filtering applications of exhaust streams, in particular as filters for diesel exhaust engines.
In an embodiment the inventive ceramic structures comprises a first phase having a stoichiometry of Cs2O.Al2O3.4SiO2 (CAS4) and a second phase having a stoichiometry Cs2O.Al2O3.2SiO2 (CAS2).
In another embodiment the inventive ceramic structures further include a third phase selected from the group consisting of SrO.Al2O3.2SiO2 (SAS2), SrO.SiO2 (SrSiO3) and combinations thereof.
The inventive structures have high thermal expansion anisotropy of between 1400-1450 ppm, as calculated from dimensional change xcex94L/L0 over a temperature range from room temperature to 1000xc2x0 C. and an average coefficient of thermal expansion from room temperature to 1000xc2x0 C. of about xe2x88x9210xc3x9710xe2x88x927/xc2x0 C. to +25xc3x9710xe2x88x927/xc2x0 C., preferably xe2x88x925xc3x9710xe2x88x927/xc2x0 C. to 15xc3x9710xe2x88x927/xc2x0 C. For the two phase embodiment a CAS4-CAS2 I-ratio, defined as the ratio the intensity of the major peak of the CAS4 phase at approximately 3.42 xc3x85 to the intensity of the major peak of the CAS2 at 3.24 xc3x85 is about 0.25 to 3.0, preferably about 0.5 to 2, and most preferably about 1.0.
An advantage of the inventive structure is its suitability in high temperature applications such as filtering particulates from diesel engine exhaust. In particular the inventive structure is especially suitable as a honeycomb diesel particular filter having an inlet end and an outlet end and a multiplicity of cells extending from the inlet end to the outlet end, the cells having porous walls, wherein part of the total number of cells at the inlet end are plugged along a portion of their lengths, and the remaining part of cells that are open at the inlet end are plugged at the outlet end along a portion of their lengths, so that an engine exhaust stream passing through the cells of the honeycomb from the inlet end to the outlet end flows into the open cells, through the cell walls, and out of the structure through the open cells at the outlet end. Diesel particulate filters having the inventive structure have been obtained.
In accordance with another aspect of the invention, there is provided a method of producing a formable mixture that involves combining a dry blend material consisting essentially of 70-90%, by weight, of a glass frit and 10-30%, by weight, Al2O3, a solvent selected from the group consisting of deionized water, an emulsion consists essentially of, about 95%, by weight, deionized water, about 0.7%, by weight, triethanolamine and about 4.3%, by weight, oleic acid, and combinations thereof, and a polymer selected from the group consisting of a crosslinked polyacrylic acid copolymer, a polyethylene oxide polymer, and combinations thereof.
Up to 30%, by weight, SrO may be substituted for Cs2O, the resulting ceramic structure then including a third phase selected from the group consisting of SrO.Al2O3.2SiO2 (SAS2), SrO.SiO2 (SrSiO3) and combinations thereof. In forming the mixture containing the glass frit having SrO substituted for cesium, a suitable polymer is an aqueous-based cellulose ether polymer selected from the group consisting of methylcellulose or hydroxylpropyl methylcellulose. Unlike the Cs2O glass frit, the Cs2Oxe2x80x94SrO glass frit has increased aqueous stability because strontium prevents the cesium from leaching and reacting with water to form a gel or cementitious mixture which is incapable of being shaped.
In accordance with another aspect of the invention, the formable mixture is shaped by extrusion to form a green monolithic structure, such as a honeycomb, which is then fired in an electric furnace at a temperature of about 1350 to 1550xc2x0 C. over a period of about 6 to 12 hours, and held at a maximum temperature for about 4 to 12 hours to form the final product structure.